Where did all these monkeys come from? – Fossil teeth may hint at an Asian origin for anthropoid primates

Where did anthropoid primates come from? This question has not been an easy one to answer. Since the early days of paleontology various experts have proposed a slew of scenarios for the origins of the primate group which today contains monkeys and apes (including us), with different experts favoring various combination of places, times, and evolutionary rootstocks. To this day questions over the who, what, when, where, why, and how of anthropoid origins continue to spur frequent debates in the pages of journals and at conferences.

A map of early anthropoid discoveries. (Keep in mind that some taxa, such as Altiatlasius from Morocco, are enigmatic and may not be anthropoids.) From Beard, 2006.

The nature of the early primate fossil record – mostly a collection of teeth and limb fragments – has made debate commonplace and consensus difficult to achieve, but during the past two decades a trickle of new discoveries has started to provide scientists with a new framework in which to consider anthropoid origins. Contrary to the hullabaloo around the fossil primate Darwinius masillae (“Ida” to her fans) last year, interdisciplinary studies have confirmed that early anthropoids were most closely related to small, tarsier-like primates and not the lemur-like adapiforms (which had been traditional favorites for the role). This recognition informs hypotheses about how early anthropoids lived and what their ancestors were like – in order to understand how early anthropoids evolved, we need to know what they originated from.

Another spot of contention has been where the first anthropoids evolved. Given that some of the earliest known anthropoids – such as the 37 million year old Biretia – have been found in Egypt and Algeria, northern Africa seemed like the logical choice for the place of the group’s origins. This hypothesis has not gone uncontested. In 1994 K. Christopher Beard and colleagues described the 45 million year old primate Eosimias from fissure-fill sites in southeastern China. Among the earliest of anthropoid primates, Eosimias was very close to the stem from which the first anthropoids radiated, hinting that the origins of the group were to be found in prehistoric Asia.

Evidence for the origin of anthropoids in Asia has continued to accumulate. In 1999 a team of paleontologists led by J.J. Jaeger described a relative of Eosimias from Myanmar they called Bahinia. It lived alongside early anthropoids such as Amphipithecus, and while it was not an anthropoid ancestor Bahinia did possess a number of archaic traits which indicated that it was a member of a long-surviving lineage which had branched off near the base of the anthropoid radiation. Its anatomy was like a time-capsule which recorded changes which occurred millions of years earlier.

More recently, Sunil Bajpai and co-authors described the 54 million year old relative of Eosimias named Anthrasimias from India. This extended the record of anthropoid primates back much further than the fossils from Africa, and last year’s description of the geologically younger (~38 million year old) anthropoid Ganlea from Myanmar by Beard and colleagues was another instance of an early anthropoid lineage branching off and persisting in Asia. The record of early anthropoids in Africa is nowhere near as rich. Prior to 40 million years ago, there is virtually nothing except a controversial taxa from Morocco called Altiatlasius which may not be an anthropoid at all. Taken all together,the fossils fall into a pattern consistent with the idea that anthropoids emerged in Asia, underwent an evolutionary radiation, colonized Africa, and then underwent a subsequent radiation.

Even fossil primates from Africa may hold clues to an Asian origin for anthropoids. Today Jaeger, Beard, and an international team of other primate specialists have published a new report in Nature which places a diverse community of prehistoric primates from Libya into a wider biogeographical pattern.

The Eocene-age fossil site located in Dur At-Talah, Libya closely approximates other fossil primate localities from elsewhere in northern Africa. At about 39 million years old, the primates from Libya would have been roughly contemporaneous with Biretia from Algeria, and the abundance of large, semi-aquatic, early elephants such as Barytherium and Moeritherium may indicate an environment similar to the slightly geologically younger primate sites of Fayum, Egypt. Also like these other sites, the Libya locality was home to multiple species of primates. Included in the assemblage – and represented entirely by teeth – was Karanisia arenula (a previously-unknown species of primate closely related to living lorises), the new species of anthropoid Afrotarsius libycus, the new anthropoid genus Talahpithecus parvus, and the already-known Biretia piveteaui, all of which were small primates which would have weighed between 120 to 470 grams in life. To put it a different way, the scientists had found a loris-like primate living alongside at least three different kinds of “dawn monkeys”, each of which represented a unique anthropoid lineage. These species were not each others closest relatives, but were disparate parts of the early anthropoid radiation.

The existence of such different primates in the same places at the same time means that a ghost lineage could be drawn from each group back to the last common ancestor of all anthropoid primates. There must be more out there than what we have been able to find so far. The question is where to look for those fossils. Citing a lack of early primate fossils from older, well-sampled localities elsewhere in northern Africa, the authors behind the new paper point to Asia. Given the number of early anthropoids and stem anthropoids which have been found in Asia it seems that not only did anthropoids originate there, but they diversified and different lineages colonized Africa at approximately the same time. Either that or there is some treasure trove of early African anthropoids that no one has seen any sign of just yet – paleontologists will have to dig deep to find the evidence to test these predictions.

Given how often news about fossil primates gets hyped and framed with “missing link” imagery, it is also worth considering how this story has hit the headlines. The paper presents a new, varied assemblage of primates contemporaneous in age with other deposits from northern Africa, and the diversity of primate species is suggestive of an earlier radiation of primates that likely happened elsewhere. The positive fossil evidence which will fill out those ghost lineages is still being sought, but the idea of an Asian origin for anthropoid primates is not new and has been seriously considered for about two decades. Nevertheless, the Daily Mail gets off to a terribly start by pulling out that old bit of bullshit boilerplate “The human family tree may have to be rewritten” in the first line of their coverage before confusing themselves by trying to tie this discovery to the later origin of hominins over 30 million years later. In contrast, Dan Vergano of USA Today presents the story well, as does Ann Gibbons at Science NOW and Ewen Callaway at the Nature “Great Beyond” blog. For once, I don’t have very much to complain about!